Electrical network



y 25, I E. D. GOODALEV I 2,354,592

ELECTRICAL NETWORKS' Filed March 26, 1942 2 Sheets-Sheet 1 V/beoreproducer' INVENTOR ELMER DUDLEY GOODALE A'FI'ORNEY July 25, 1944. E.D. GOODAEE 2,354,592

' ELECTRICAL NETWORKS Filed March 2a, 1942 2 Sheets- Sheet 2 wCR o 0.05010 0.15 0.20 0.25 0.30 INVENTOR m ELMER DUDLEY 600pALE ATTORNEY Fi .5.v BY #394411 Patented July 251944 ELECTRICAL NETWORK Elmer DudleyGoodale, Bayside, Long Island,

assignor to Radio Corporation of America, a corporation of DelawareApplication March 26, 1942, Serial No. 436,267 (01. 119-171) Claims.

This invention relates to'thermionic amplifiers and, more particularly,to thermionic amplifiers for use in television and video systemsrequiring a substantially uniform amplification over a very wide band offrequencies.

In television and video systems, as well as other applications, it isoften necessary to amplify electrical currents uniformly over a wideband of frequencies extending from very low frequencies to frequencieson the order of five megacycles or more. In certain of the apparatus, asin television, it is also important that the amplified currents retainthe same relative phase as the input currents possessed in order toavoid phase'distortion. If the amplification of the signalling currentsis not uniform, or if phase distortion is present, then the quality ofreproduction of the picture is reduced, becoming blurred, distorted,

or giving evidence of ghost images.

Thermionic amplifiers, due to their inherent .plate-to-cathodecapacities and grid-to-cathode capacities, militate against uniformamplification over a wide band of frequencies, since this capacity actsas a shunt circuit whose impedance varies inversely with frequency. Toovercome the deleterious effects of this shunt capacityin the platecircuit, it has been proposed to use peaking" circuits in which a smallinductance is incorporated so that at the very high frequencies itsimpedance, when taken together with the impedance of the plate-cathodecircuit, tends to resonate and so maintain the load impedance at areasonably constant figure over a large percentage of the band offrequencies to be amplified.

The common types known to the art are the shunt peaking," the "seriespeaking," and the combined shunt and series peaking circuits. All ofthese circuits, however, have certain shortcomings. For example, in"shunt peaking" the distributed capacities across the coil and theresistance included in the plate circuit are neglected. In manyapplications the neglect of these stray capacities results'in .aresponse "characteristic which deviates widely from that of thecalculated response. The same is true in the series peaking" circuits,where again it is usual to disregard the stray capacities of theplatecircuit elements. Ordinarily, if not too many stages are to be used incascade, reasonably good'results may be obtained in television receiversusing these priorly known types'of high frequency correctioncircuits,However, in transmitter application, where it is necessary, due-to highpower present, to use water-cooled resistors, the distributed capacitybecomes so large that to neglect it results in unsatisfactory responsecharacteristics of the transmission of video signals.

To overcome the shortcoming of correction circuits known in the priorart, I have found that improved results can be obtained by taking intoaccount the stray capacity of both the inductance and the resistanceused in the plate circuit and by suitably proportioning the values ofthese c'apacities with respect to the sum of the platecathode capacityof the tube, the grid-cathode capacity and stray capacity due to thewiringof the succeeding stage, that it is possible to obtain a, responsecharacteristic which will fall 015i smoothly or monotonically only 1.3%over the entire pass band from a uniformvalue and in which the delaytime of the maximum frequency is held within such a small value that thephase .response can be considered substantially linear. 'I'hese resultsare obtained by suitably relating the three capacities concerned as wellas the value of inductance and resistance and my. invention willdescribe how these parameters must be related.

Accordingly, one of the objects of my invention is to provide a new andimproved thermionic amplifier having a superior response and phasecharacteristic. V

Another object of my invention is to provide a thermionic amplifierwhich is corrected so as to give substantially uniform amplifications atvery high frequencies as well as low and moderate frequencies.

Still another object of my invention-is to-provide a thermionicamplifier having a load circuit comprising a resistance and aninductance connected in series and each of these elements shunted bysuitable capacities to, provide substantially constant impedance over awide band of frequencies.

Other objects will become apparent upon reading the following detaileddescription in conjunction with the drawings. I 4

In the drawings, in Figure 1, I have shown a circuit of a thermionicamplifier embodying my invention, using shunt circuit elements;

In Figure 2 I have shown electrical network which is the equivalent ofthe plate circuit of the thermionic amplifier shown in name V f InFigure 3 I have shown a further modificator and video reproducer;

In Figure 4 I have also showna further eiucapacities across plate.

schematically an bodiment of my invention in which the electricalnetwork of my invention is used in a modu- In Figure 1, the thermionicamplifier 3 is fed with input energy to its grid-cathode circuit throughthe terminals I. A suitable biasing battery 1 is provided to'operate theamplifier on the linear portion of its characteristic. The plate circuitcomprises a serially connected load resistor l1 and inductance l3shunted by condensers l9 and II respectively. In addition; there isshown the condenser II which represents the plate-cathode capacity ofthe tube as the distributed capacity of the wiring and the grid-cathodecapacity of the amplifier 5. The tube 3 is energized by a source ofvoltage 9 and the tube is energized by input signals from the outputcircuit of the tube 3 through the conof one of thelaswell.

denser 2| and the resistor 23. It is important to note that both theinductance l3 and the resistor ll are shunted by individual condensersbecause it is these condensers which make it possible to achieve theimproved performance of the amplifier. It is of course of furtherimportance to be sure that the condensers l5. l9, and II, all I bearproper-relation to each other and to the inductance and resistance inorder that the amplification shall be uniform. To appreciate and to showthe relationship between these condensers it is necessary to. digressand analyze the electrical network in the'plate circuit of the tube 3.

The equivalent network of the tube 3 plate circuit is shown in Figure 2,in which the condenser C is the equivaient of the condenser II in Figure1, while condensers C1 and C2 are the equivalent of condensers l5 andI9, respectively, and L and Rare the equivalent of the elements l3 andII, respectively, in Figure 1. It is well known of course that theamplification of a thermionic amplifier whose plate impedance is largeperformance, it somewhat more convenient to find the impedance as afunction of the angular velocity. This can be done by substituting itfor p. In addition, if we let L=GCR, 01:170. Ca=mC, then by substitutionand collection of like terms, we can find the ratio of the impedance tothe resistance R fromthe expression:

(2) z, [1-arllbC'1F-MGMC'EHJMCR jBy further substitution of z=wCR, wederive:

which is now in the form to apply the Tellegen- Verbeck procedure forascertaining the values of a, b, and m, which will make the squaredimpedance function given by the Equation 4 a monotonic one.

A monotonic solution is one in which the impedance function changes uniformly without change of sign of the rate of change of the impedancewith frequency. In accordance with the method for solving for the valuesof a, b, and m to give such a montonic solution, it is necessary that.the co-efilcients of the same powers of a: occurring in the numeratorand the denominator be equated, that is to say, that the coefiicient of1 in the numerator is equated to the co-eflicient of a in thedenominator, the coefllcient of x in the numerator is equated to theco-eflicient of 2: occurring in the denominator, etc., and thus resultsa series of simultaneous linear equations which, when solved, will givecompared to the impedance of the plate load circuit is given by theproduct of the transconductance of the tube and the load impedance.Since the transconductance of the tube is substantially constant, itwill'be readily noted that constant amplitude as a function of frequencyrequires that the impedance remain constant. that is to say, that theimpedance of the network pedance of the network as a function of irequr.

. The impedance Zo of the network shown in Flgure2canbeshowntobe,m

This form of' theequation, when solved, provides the reciprocal of theindicial admittance, but

sincewe are interested only in the steady-state operational form,

' Z0 should be substantially constant over the band the values of a, b,and m, to render the impedance-squared function of Equation 4, one whichhas constant value over a prescribed range of frequencies.

The resultant equations are as follows:

Reducing these equations we obtain:

a=-(1m)1/2(1+ m) (l+m)(a-cm-2m) vcircuit, and if b is made 0, ,then thecircuit reduces 'to the xso-called resistance-capacity shunt-peaking"circuit. By substituting m equal to 0 in Equations 8 and 9, we find thatthe values of a and b are 0.414 and 0.353, respectively. By nowsubstituting b equal to o "in-the same equations, we find that 4: equals0.736 and m 0.269. Since it is known that the two limiting cases canprovide substantially monotonic solutions, it will be noted m may have arange of values lyingbetween and 0.269. Thus, if we restrict the valueof m between these values, we may arbitrarily select m within the rangeand solve for a and b with the knowledge that any one of these solutionswil give a substantially monotonic solution over the required frequencyrange. A table of values showing the relationship between a, b, and 'mis shown in the following table:

m a b 0.000 0. 414 0.30s 0.050 0.404 0. 271 0.100 0. m0 0. m 0. 100 0.use 0. 1a4 0.200 0.040 0.012 0.200 0.730 0.000

, represents the response for the indicated value of m-. There is alsoshown, in Figure 5, the maximum time delay at the maximum frequency forwhich substantially uniform amplification is desired as a function of m.It will be notedthat as m is made larger the maximum time delaydecreases.- Although the decrease in maximum time delay with increasingvalues of m is not a rapid one, it affords the designer some flexibilityin choice of circuit constants so that, depending upon whether or notthe cutoff of the amplifier or the phase delay of the amplifier isimportant for the specific problem at hand, the value of m may be chosenwhich will give the optimum solution. The time of transmission isobtained by noting that the tangent of phase angle isequal to the ratioof the imaginary portion of the impedance function to the real'portionof the impedance function. From this expression, the phase angle can bewritten as the anti-tangent of the ratio. Differentiating the phaseangle with respect to the variable it will then result in an expressionwhich gives the timeof transfeeds the reproducers synchronizing circuit41, which may be 01' any of the types well known in 'the art, to supplythe synchronized sweep frequencies for the video reproducer. It will benoted that the synchronizing separator 45 is not connected across the,terminals ll as is done in conventional receivers. The reason for thisis,

that when the synchronizing separator is fed in parallel with the videoreproducer, the com-' bined stray ca'pacity becomes approximately twiceas. large as that of the stray capacity of the video reproducer itself.Under these conditions, therefore, the value of the resistance It wouldhave to be reduced to approximately one half the value to obtain thedesired band width of frequencies. Consequently, the amplifier wouldhave its efiiciency reduced.

However, by placing the synchronizing separator across the resistor 4|,the total stray capacity is reduced, since the input capacity of thesynchronizing separator 45 may actually supply the required capacityacross the resistor, that is to say, the input. capacity of thesynchronizing separator supplies the capacity 43. It is possible to'usethis type of connection because the band width required for thesynchronizing signalis considerably smaller than that required for thevideo amplifier. One maytherefore use a much larger value of resistancein the circuit shown in Figure 3 and consequently achieve 7 much highergain and greater efllciency from the amplifier. 53.

Another example of where this circuit is particulariy useful is shown inFigure 4,.where the network of the type shown in Figure 2 is embodied ina modulator stage of a television transmitter. In Figure 4 the signalsto modulate the carrier frequency are applied to the terminals 5| of themodulator tube 53 whose plate circuit comprises the inductance j 51 andthe watercooled resistor 59. The output of the tube 53 feeds through acondenser 6'! which is supplied with a biasing rectifier 89 to thepush-pull. stage of the transmitter. Coils II are radio frequency coilsto. prevent the carrier frequency from geting back into the modulatorstage. The carrier mission, since by definition this time oftransmission is equal to the rate of change of phase with respect to thevariable 2. The time delay is equal to the difierence between the timesof transmission at the top frequency and minimum frequency. The resultsof the maximum time .delay for the type of circuitlchosen areincorporated in the curve shown in Figure 5.

.The network shown in Figure 2 becomes particularly useful in certaintelevision applications, of which the circuit shown in Figure 3 isexemplary. In Figure- 3 there is shown a video amplifier circuit such asis useful in the tele-- vision receiver. The thermionic amplifier It isfrequency is suppliedto a balanced input circuit comprising theinductance II and the condensers 13 and 11 and is tuned by the condenseris. The push-pull tubes BI and I3 consequently have their grids outsidethe radio frequency in pushpuli, while the modulator merely serves toshift the operation point on the characteristic cf'the two tubes so asto vary the amplitude of the radiated radio frequency energy inaccordance with the video signals supplied in the terminals II. In atransmitter using water-cooled coil andresistance, the distributedcapacity may run on the order of 25 micro-micro farads across theresister and as high as micro-micro farads across the coil. With thesevalues it will be readily appreciated that-th distributed capacityacross the inductance and across the resistance fed with the detectedcombined video and synchronizing signals at the terminals IL. The outputcircuit comprising the lumped stray capaci ties 35 and the load resistor4] with its peaking coil 31 and capacities 43 and II, respectivelyfeedsboth the video reproducer from the output terminals 48 and thesynchronizing signal separator 45 which is'connected across the resistorii, The output of the synchronizing separator I were necessary in theconventionally designed types of modulators.

It will be appreciated that the applications shown in Figures 3 and 4are by way of example only and are not exclusive of use of this type ofcircuit in other portions of television systems or in amplifierswherever a wide band of frequencies must be transmitted and in which theamplitude response must be substantially constant over the band offrequencies.

Various alterations and modifications of the present invention maybecome apparent to those skilled in the art and it is desirable that anyand all such modifications and alterations be considered within thepurview of the present invention except as limited by the hereinafterappended claims.

trode, and an anode, and a circuit comprising an inductance shunted by acondenser connected in 1 series with a resistance shunted by a condenserconnected in the cathode-anode circuit, the value of said inductancebeing chosen proportional to the product of the resistance squared andthe distributed capacity between the anode and cathode of the saidthermionic amplifier.

3. A video amplifier for amplifying a wide band of frequenciesuniformly, said band having a maximum predetermined frequency comprisinga thermionic amplifier having at least a.

cathode, control electrode, and an anode, and a circuit comprising aninductance shunted by a condenser connected in,series with a resistanceshunted by a condenser-connected in the oathode-anode circuit, saidresistance having a value inversely proportional to the product of saidmaximum predetermined frequency and the distributed capacity between theanode and cathode of said thermionic amplifier.

4. A video amplifier for amplifying a side band of frequenciesuniformly, said band having a maximum predetermined frequency comprisinga thermionic amplifier having at least a cathode, control electrode, andan anode, and a circuit comprising an inductance shunted by a con-Vdenser connected in series with a resistance shunted by a condenserconnected in thecathode-anode circuit, said resistance having a valueinversely proportional to the product of said predetermined maximumfrequency and the distributed capacity between the anode and cathode ofsaid thermionic amplifier and the value of said inductance being chosenproportional to the product of the square of the resistance and thedistributed capacity between said anode and said cathode.

5. A video amplifier for amplifying a side band of frequenciesuniformly, said band having a maximum predetermined frequency comprisinga thermionic amplifier having at least a cathode,

control electrode, and an anode, and a circuit comprising an inductanceshunted by a condenser connected in series with a resistance shuntedby acondenser connected in the cathode-anode circuit, said resistance havinga value inversely proportional to the product of said maximumpredetermined frequency and the distributed capacity between the anodeand cathode of said thermionic amplifier, the value of said inductancebeing chosen proportional to the product of the square of the resistanceand the distributed capacity between said anode and said cathode, saidcapacity shunted across said inductance being proportional to saiddistributed capacity, said proportionality factor being greater thanzero and less than 0.353 and said capacity shunted across said resistorbeing proportional to said distributed capacity, said proportionalityfactor being greater than zero and less than 0.269. v

Y EIMER DUDLEY GOODALE.

